The present invention relates to an apparatus and method for implementing a return line of the type which are used with probe stations and, in particular, to an adjustable strap implemented return line of the type used, for example, to electrically interconnect the signal probe and ground probe of a high frequency probe station as well as a procedure for making the strap.
The testing of high-frequency electrical components is typically performed at a probe station. At the probe station, the device-under-test (DUT) is held temporarily, such as by vacuum pressure, on a table or so-called stage. The signal probe and ground probe are normally mounted above the stage, each by its own X-Y-Z positioner, so that each probe is capable of independent X-Y-Z movement across the measurement plane defined by the top surface of the device. This allows the tip of each probe to be moved between various measurement sites or ground sites on the DUT.
To enable proper transmission of high-frequency signals through the signal probe, the tip of the signal probe usually comprises a controlled-impedance structure having a signal line conductor and ground line conductor in predetermined closely-spaced arrangement to each other. In order to return the ground line conductor of the signal probe to the ground of the DUT, a separate return line is used since the spacing between the measurement site and the nearest ground site on the DUT is typically variable and often is much larger than the spacing between the signal line and ground line conductors on the tip of the signal probe. The desirability of using a sheet-like strap as a return line for a high-frequency probe is recognized in several references including Gleason et al. U.S. Pat. No. 5,045,781, Soar U.S. Pat. No. 4,838,802, and Japanese Patent Publication No. 1-178872 by Chiyojima published Jul. 17, 1989.
Gleason shows a rigid strap extending in cantilevered position from the ground probe for wiping engagement across the outer conductor of the signal probe. In this setup, typically there will be an unused portion of strap extending past the signal probe where the length of this unused portion is many times larger than the tip-to-tip separation between the probes. This occurs because the majority of probing is typically performed close-in, that is, with the tips of the signal and ground probes separated by distances which do not exceed 0.010 inches, whereas the length of the rigid strap is not allowed to be any shorter than the maximum anticipated distance between the signal and ground probes, which can be one inch or more. This means that noisy components that are relatively far removed from the vicinity of the measurement and ground sites can still be near enough to the unused portion of the strap so that spurious energy leaks into the strap and impairs the accuracy of the readings which are being taken. This also means that if the vicinity surrounding the test and ground sites is cluttered with high-profile components, there is a strong possibility that the unused portion will interfere physically with these components to prevent positioning of the signal and ground probes on their respective sites.
Soar shows a fixed-length flexible ground strap permanently connected at each end to a respective probe. Because of its flexibility, the strap used in Soar arches outwardly when the signal and ground probes are brought together for close-in probing. Like the unused portion of the rigid strap, this arched portion can pick up noise from devices which are relatively far away and can interfere with surrounding components when attempts are made to move the probes to certain of their positions. Furthermore, if the signal and ground probes are inadvertently moved apart in the X-Y plane by a distance exceeding the length of the strap, this can result in a broken probe tip or can cause a hardened tip to be dragged across the device causing damage to the device surface. Moreover, the Soar return line does not permit the Z-axis or vertical position of each probe to be varied independently. If, for example, the signal probe is lifted over a high profile component and brought back down to a new measurement site, it is also necessary to lift and then reposition the ground probe.
Chiyojima shows a roll-out strap assembly where the roll-out mechanism includes a reel mounted rotatably about the signal probe and around which the flexible strap is wound. The assembly also includes a protective enclosure, which houses the reel and strap, and a take-up mechanism comprising a crank geared with the reel so that when the crank is turned, the strap is rewound about the reel for close-in probing.
Although, in Chiyojima, operation of the crank keeps the length of the strap to a minimum, if the operator accidentally applies too much force to the crank, this can drag the tip of a probe across the device or can cause the strap to break clear of its protective enclosure so the enclosure must be taken apart in order to make repairs. The same problems occur if the signal and ground probes are separated by a distance in the X-Y plane which exceeds the maximum length of the strap. Additionally, the overall assembly has many parts that require careful fitting, so that the interface is expensive to assemble and not readily adaptable to preexisting probe systems. Furthermore, because the strap is permanently connected at each of its ends to a respective probe, both probes must be repositioned whenever there is a vertical or Z-axis movement by one of the probes. Finally, the wound portion of the strap and its bulky enclosure obscures the view that looks directly down at the probes so that during close-in probing, it is often impossible to confirm the precise position of the tip of the signal probe.
Accordingly, an object of the present invention is to provide an improved return line system for high frequency probe stations.
A related object of the present invention is to provide a return line which has minimal sensitivity to noise that occurs outside the immediate probing vicinity yet which is cost-effective to produce.
Another related object of the present invention is to provide a return line in which a signal probe and ground probe are reliably connected electrically yet excessive X-Y plane separation between the probes will not result in damage to the probes or the device under test.
Still another related object of the present invention is to provide a return line which makes possible replacement of worn signal or ground probe tips without replacement of the entire probe head assembly including the X-Y-Z positioners.
Yet another related object of the present invention is to provide a return line that enables the signal probe tip to be lifted over a high-profile component and brought back down to a new measurement site without the need to also reposition the ground probe.
Yet another related object of the present invention is to provide a return line that enables close-in probing even when the surface of the device is cluttered with high-profile components.
Yet another related object of the present invention is to provide a return line which is readily adaptable to preexisting systems.
Yet another related object of the present invention is to provide a return line which accommodates quick travel by the signal probe between different measurement sites.